1. Field of the Invention
The present invention relates to focus control in an image-taking apparatus.
2. Description of the Prior Art
A so-called TV-AF method is mainly used for auto-focus (AF) in video cameras. In the TV-AF method, an object image is photoelectrically converted by an image-pickup element or the like into a video signal from which a high-frequency component representing the sharpness of the video is extracted through a band pass filter, and the drive of a focus lens is controlled to search for the position at which an AF evaluation value indicating the sharpness is at the maximum. As shown in FIG. 5, the position at which the AF evaluation value is at the maximum corresponds to an in-focus position.
When focusing is thus achieved, the AF evaluation value at that point is stored. Then, an AF evaluation value taken after the focusing is compared with the stored AF evaluation value (that is, determination of restart is made), and if a difference greater than a predetermined level exists between them, AF control with the TV-AF is restart (re-performed).
In this manner, in the TV-AF method for video cameras, the drive of a focus lens is controlled through repeated cycles of start of AF control, determination of restart, restart of AF control, and determination of restart, thereby achieving control such that the AF evaluation value maintains the maximum in taking moving images.
The AF method includes an external distance measuring method which uses an external distance metering sensor. The method includes, for example, a passive type in which a luminous flux from an object is split into two which are received by two light receiving sensors (line sensors), respectively, and the correlation of signals from the two sensors are calculated. Then, calculations are made to find how many pixels on the line sensors should be shifted to provide the highest correlation. Distance information can be obtained on the basis of the shift amount by using the principles of triangulation. The method also includes an active type in which an ultrasonic sensor is used to measure the distance to an object from the propagation speed of ultrasonic waves. In another active type, an infrared sensor is used to measure the distance to an object with triangulation.
Another AF method is called a TTL (Through the Taking Lens) phase difference detection method. In the phase difference detection method, a luminous flux passing through the exit pupil of an image-taking lens is split into two which are received by a set of light receiving sensors. Then, a displacement amount in signals (phase difference signals) output in accordance with the received light amounts, that is, a relative positional displacement amount in the direction in which the luminous flux is split, is detected to determine a defocus amount of the image-taking lens. A drive amount and a drive direction to reach an in-focus position of a focus lens are directly determined on the basis of the defocus amount. In this manner, in the phase difference detection method, once the light receiving sensors are used to perform electric charge storing operation, the drive amount and the drive direction of the focus lens to reach the in-focus position can be provided, so that focus adjustment operation can be performed at high speed.
Japanese Patent Application Laid-Open No. 2002-258147, Japanese Patent Application Laid-Open No. H05(1993)-64056 and the like have proposed methods in which the AF is performed by combining the aforementioned external distance measuring method or phase difference detection method with the TV-AF method. In these proposals, the external distance measuring method or the phase difference detection method is used to calculate the drive amount and drive direction of a focus lens to reach an in-focus position to move the focus lens to the in-focus position, and then the TV-AF method is used to correct focus to precisely achieve focusing.
As described above, in the TV-AF method, restart is performed when the AF evaluation value is changed even after an in-focus state is achieved. The same processing operation is performed whether the change of the AF evaluation value is large or small. If the distance to an object is largely changed, it may take a long time to attain an in-focus state.
When the distance to an object is changed but the AF evaluation value shows no difference, for example in the case where a lens is focused on a subject different from a true object at a changed distance, the TV-AF is not restarted for the true object and the in-focus state may not be maintained.